This invention relates to optical scanning, e.g., scanners for reading bar codes.
Targets having indicia of different light reflectivity, such as bar code symbols, have been scanned by directing a laser beam along an optical path to a symbol located in the vicinity of the reference plane lying generally normal to the optical path. The laser light reflected off the symbol is detected by a detector having a finite field of view or scanning field. (The field of view is also referred to as the scanning area, especially in the case of two-dimensional bar code symbols, such as the PDF417.)
In the case of hand-held scanners, a scanner must be manually positioned relative to an object having a symbol in such a way as to ensure that the scanner's scanning field encompasses the symbol (or enough of the symbol to enable its decoding).
In many applications it is desirable to provide hands-free scanner operation, for example, in point-of-sale check-out systems of the type generally found in supermarkets. Hands-free operation is achieved in a number of ways. For example, in the case of point-of-sale checkout systems, it is well-known to mount symbol scanners underneath check-out counters. (See, for example, U.S. Pat. No. 3,978,317.) Portable, hand-held scanners can also be temporarily mounted (e.g., in a mounting stand as is shown in U.S. Pat. No. 4,766,297).
To scan a symbol using fixed-position or temporarily mounted scanner environments, the object having the symbol to be scanned must be manually positioned relative to the fixed-position scanner in order to move the symbol into the scanner's field of view.
It is clearly advantageous to maintain a scanner in a non-scanning (low-power or off) mode when it is not in use. Among other reasons, this reduces power consumption and prolongs the life of the scanner.
In order for a scanner that is in a non-scanning mode to scan and decode a symbol in its field of view, some means of initiating a scanning episode must be provided. (A scanning episode may include, for example, propagating and directing a light beam towards the symbol to be scanned, detecting at least a portion of the light of variable intensity reflected off the symbol over the field of view, generating an electrical signal indicative of the desired light intensity, processing the electrical signal into data descriptive of the symbol being read, and decoding the data to obtain the values which the symbol represents.)
Conventional, hand-held scanners are typically hand-gun shaped and have a manual trigger for initiating scanning episodes. Some systems may use a two-phase triggering mechanism, in which a first phase is used to aim the scanner's beam at the target symbol and a second phase is used to initiate the scanning episode.
To retain hands-free operation of mounted scanners, it is useful to have an automatic triggering mechanism in order to initiate a scanning episode.
McMillan (U.S. Pat. No. 4,766,297) describes a dual mode stationary and portable scanning system. McMillan's stationary mode places the scanner in a fixed, non-adjustable position in a support fixture, thereby positioning it to have a fixed scanning area. In its stationary mode, the scanner may be triggered by interrupting a beam between two fixed locations on the scanner's support fixture.
A tilting, wall mountable bar code scanner with an adjustable, targetable scanning area is described in the application of Charych, U.S. application Ser. No. 08/059,822, filed on even date herewith, entitled Tilting Wall-Mounted Optical Scanner (incorporated herein by reference). In such a system the target area is not fixed.